This invention relates to a method for reducing rub-off from a substrate, such as paper, having a toner image on at least one side of the substrate by depositing a plurality of dots of a substantially clear phase change composition on the side of the substrate bearing the image with the dots cumulatively covering an area of the substrate bearing the image sufficient to reduce rub-off from the substrate. This invention further relates to the use of a phase change composition deposited on toner or printed images on a substrate to prevent rub-off from the substrate.
In electrophotographic copying, digital copying, and printing processes, images are formed on selected substrates, typically paper, using small, dry, colored particles called toner. Toner usually comprises a thermoplastic resin binder, dye or pigment colorants, charge control additives, cleaning aids, fuser release additives and optionally, flow control and tribocharging control surface treatment additives.
The thermoplastic toner is typically attached to a print substrate by a combination of heating and pressure using a fusing subassembly that partially melts the toner into the paper fibers at the surface of the paper substrate. Additionally, the fused toner image surface finish can be controlled by the surface finish on the surface of the fuser roller. Thus, the gloss of the image may be controlled between diffuse (low gloss) and specular (high gloss). If the surface finish of the image is rough (diffuse) then light is scattered and image gloss is reduced.
Typically, in an electrophotographic printer, a heated fuser roller is used with a pressure roller to attach toner to a receiver and to control the image surface characteristics. Heat is typically applied to the fusing rollers by a resistance heater such as a halogen lamp. Heat can be applied to the inside of at least one hollow roller, and/or to the surface of at least one roller. At least one of the rollers is typically compliant. When the rollers of a heated roller fusing assembly are pressed together under pressure, the compliant roller deflects to form a fusing nip. Most heat transfer between the surface of the fusing roller and the toner occurs in the fusing nip. In order to minimize xe2x80x9coffset,xe2x80x9d which is the amount of toner that adheres to the surface of the fuser roller, release oil is typically applied to the surface of the fuser roller. Typically, the release oil is silicone oil plus additives that improve attachment of the release oil to the surface of the fuser roller, and dissipate static charge buildup on the fuser rollers or fused prints. Some of the release oil becomes attached to the image and background areas of the fused prints.
Fused toner images can be substantially abraded or xe2x80x9crubbed-offxe2x80x9d by processes such as duplex imaging, folding, sorting, stapling, binding, filing and the like. Residue from this abrasion process causes objectionable and undesirable marks on non-imaged areas of adjacent pages or covers. This process, and image quality defect, are known as xe2x80x9crub-offxe2x80x9d and exist to varying extents in many electrophotographic copies and prints. The basic xe2x80x9crequirementsxe2x80x9d for generation of rub-off are a donor (toner image), a receptor (adjacent paper page, envelope, mailing label, etc.), a differential velocity between donor and receptor, and a load between donor and receptor.
In general, mechanisms of rub-off are consistent with those of abrasive and adhesive wear mechanisms. Relevant factors include: toner toughness, toner brittleness (cross-linking density), surface energy or coefficient of friction of the toner, adhesion of the toner to the paper substrate, cohesive properties of the toner itself, the surface topography of the toner image, the level of load and the differential velocities of the wearing surfaces. Some of these factors are under the control of the machine and materials manufacturers, and some are under the control of the end user.
Toner rub-off may be reduced by the use of tougher toner, lower surface energy toner materials (resulting in lower coefficient of friction), better-fused toner, and a smoother toner image surface finish (but this increases image gloss.)
Unfortunately, there are undesirable consequences associated with each of the above rub-off reduction factors. A tougher toner is more difficult to pulverize, grind, and classify which increases manufacturing costs. Additionally, smaller toner particle size distributions are more difficult to achieve with tougher toner. Adding wax to the toner may provide additional release properties from the fuser roller surface, and add lubrication to the surface of the toner, but triboelectric charging behavior may be adversely affected. A more easily fusible toner may create more toner offset to the surface of the fuser rollers, or increase the tendency of fused prints or copies to stick together in the finisher or output trays. Creating a more specular (smoother) image surface finish increases image gloss, which may be objectionable in some applications. Fuser release oil can lower the coefficient of friction of the fused image, but this effect is temporary since the oil is adsorbed into the paper substrate over time. Fuser release oil can also cause undesirable effects in the rest of the electrophotographic process, especially in duplex printing operations. Extensive efforts have been directed to the development of improved methods for reducing rub-off without modification of the fusing process.
According to the present invention, rub-off from a substrate bearing a toner image is reduced by a method for reducing rub-off from a substrate having a front side and a back side and bearing a toner image on its front side, the method comprising: depositing a substantially clear phase change composition on the front side of the substrate as a plurality of dots, the dots cumulatively covering an area of the front side sufficient to reduce rub-off from the front side.
The invention further relates to a method of reducing rub-off from a substrate having a front side and a back side and a plurality of printer or digital copier produced toner or printed images on its front side, the method comprising: depositing a substantially clear phase change composition on at least a portion of the toner or printed images as a plurality of dots, the dots cumulatively covering an area of the toner or printed images sufficient to reduce rub-off from the front side.